Air Pollution Impacts

Isle Royale National Park

Natural and scenic resources in Isle Royale National Park (NP) are susceptible to the harmful effects of air pollution. Mercury (and other air toxics), nitrogen, sulfur, ozone, and fine particles impact natural resources such as wildlife, surface waters, and vegetation, as well as visibility. Click on the tabs below to learn more about air pollutants and their impacts at Isle Royale NP.

Toxics, including heavy metals like mercury, accumulate in the tissue of organisms. When mercury converts to methylmercury in the environment and enters the food chain, effects can include reduced reproductive success, impaired growth and development, and decreased survival. Other toxic air contaminants of concern include pesticides (e.g., DDT), industrial by-products like PCBs, and emerging chemicals such as flame retardants for fabrics (PBDEs). Some of these are known or suspected to cause cancer or other serious health effects in humans and wildlife.

Concentrations of ammonium, an N compound and indicator of nearby agricultural activity, have increased in precipitation in the Great Lakes region in recent decades. During that same period, declines in nitrate concentrations have been observed and as a result, total nitrogen deposition remains elevated above natural conditions and relatively unchanged (NPS 2010 [pdf, 2.8 MB]; Lehmann and Gay 2011; Lehmann et al. In Prep). Sulfur emissions and resultant sulfate concentrations in precipitation have gone down more significantly in recent decades due to air pollution controls (Lehmann and Gay 2011). However, sulfur remains a concern at Isle Royale NP because it plays an essential role in the methylation of mercury, leading to toxic accumulation of methylmercury in fish and wildlife.

How much nitrogen is too much?

Nitrogen (N) is a fertilizer and some nitrogen is necessary for plants to grow. However, in natural ecosystems, too much nitrogen can disrupt the balance of plant communities, allowing weedy species to grow faster at the expense of native species. As a result, biodiversity may be lost. For example, experiments show that increased nitrogen increases the risk of extinction of rare wetland species like the Sarracenia purpurea (pitcher plant) (Gotelli and Ellison 2002). N deposition may also upset the ecological balance of boreal lakes at Isle Royale NP. A project is underway to examine the effect of excess nitrogen on lakes in the park, and determine whether the critical load has been exceeded. In N-sensitive, high elevation lakes in the Rocky Mountains, this shift to a more disturbed, polluted system occurred at a nitrogen wet deposition loading of about 1.5 kilograms per hectare per year (kg/ha/yr) (Saros et al. 2010). Nitrogen wet deposition at Isle Royale NP is estimated between 3–4 kg/ha/yr (NADP 2010 [pdf, 1.9 MB]) and nitrogen may already be causing changes to park lakes. Critical loads for lakes can be used to establish goals for ecosystem recovery.

While ground-level ozone concentrations are generally low at Isle Royale NP, Michigan, there are a few ozone-sensitive species present in the park, such as Apocynum androsaemifolium (Spreading dogbane).

Naturally-occurring ozone in the upper atmosphere absorbs the sun’s harmful ultraviolet rays and helps to protect all life on Earth. However, in the lower atmosphere, ozone is an air pollutant, forming when nitrogen oxides from vehicles, power plants, and other sources combine with volatile organic compounds from gasoline, solvents, and vegetation in the presence of sunlight. In addition to causing respiratory problems in people, ozone can injure plants. Ozone enters leaves through pores (stomata), where it can kill plant tissues, causing visible injury, or reduce photosynthesis, growth, and reproduction.

There are a few ozone-sensitive plants in Isle Royale NP including Apocynum androsaemifolium (Spreading dogbane), Ascelpias syriaca (Common milkweed), and Prunus serotina (Black cherry). Because ozone levels at the park are low, there is a low risk of ozone injury to plants (Kohut 2004 [pdf, 187 KB]). A review of monitoring results found no ozone injury to plants in regions near Isle Royale NP (Swackhamer and Hornbuckle 2004 [pdf, 4.5 MB]).

Visitors come to Isle Royale NP to enjoy the spectacular remote islands in the vastness of Lake Superior, with forests, inland lakes, wolves, moose, and other features that comprise this rugged wilderness. Unfortunately, park vistas are sometimes obscured by haze caused by fine particles in the air. Many of the same pollutants that ultimately fall out as nitrogen and sulfur deposition reduce visibility by contributing to this haze . Additionally, organic compounds, soot, and dust reduce visibility. Smoke from nearby forest fires also contributes to particulate matter in the region.

Visibility effects at Isle Royale NP include:

Reduced visibility, at times, due to human-caused haze and fine particles of air pollution, including dust;

Reduction of the average natural visual range from about 110 miles (without pollution) to about 60 miles because of pollution at the park;